1. Field of the Invention
The present invention relates to a polymer composition for a conductive roller, a polymer composition, a conductive roller, and a conductive belt. More particularly, the conductive roller and the conductive belt are used for a conductive mechanism of an electrophotographic device of office appliances such as a copying machine, a printer, and the like. The polymer composition can be used effectively for conductive rollers, having a low electric resistance value, such as a developing roller, a charging roller, and a transfer roller for a color copying machine and a color printer. The present invention also relates to improvement of the state of the polymer composition when it is foamed to form the conductive roller such as the transfer roller, the developing roller, the charging roller, and a toner supply roller thereof and productivity of the conductive roller.
2. Description of the Related Art
It is necessary to impart an appropriate stable electric resistance value to the developing roller, the charging roller, and the transfer roller for use in the color copying machine and the color printer. To impart conductivity to the roller of this kind, the following two methods are conventionally used: Used in one of them is an electroconductive polymer composition containing a conductive filler such as powder of metal oxide and carbon black in a polymer thereof. Used in the other of them is an ionic-conductive polymer such as urethane rubber, acrylonitrile butadiene rubber (NBR), and epichlorohydrin rubber.
In the case where the electroconductive polymer composition is used, there is a region in which an electric resistance changes rapidly owing to a slight change of the addition amount of a conductive filler. Thus it is very difficult to control the electric resistance. In addition, because it is difficult for the conductive filler to disperse uniformly in the polymer composition, an electric resistance value has variations in the circumferential and widthwise directions of the roller.
The electric resistance value of the conductive roller composed of the electroconductive polymer composition containing the conductive filler depends on an applied voltage and does not have a constant electric resistance value. In particular, in the case where the carbon black is used as the conductive filler, there is no stable correlation between the addition amount of the carbon black and the volume resistivity of the polymer. Moreover as described above, there is the region in which the electric resistance changes rapidly owing to a slight change of the addition amount of the conductive filler. Therefore these tendencies appear outstandingly.
The conductive roller composed of the electroconductive polymer has variations in its electric resistance value, as described above. Thus as the recent tendency, a roller composed of the ionic-conductive polymer is used more than a roller composed of the electroconductive polymer in a copying machine and a printer required to have a high image quality by using digital image processing technique and color image processing technique and save energy.
However, it is difficult to lower the electric resistance value of the ionic-conductive polymer composition. Thus the ionic-conductive polymer composition having a volume resistivity value less than 108.0 [Ω·cm] has not been realized in the mode in which it can be put into practical use.
More specifically, the volume resistivity value of urethane is more than 108.5[Ω·cm] in the case where no additive is added thereto. Attempts of increasing the ionization degree by adding various metal ion salts or a quaternary ammonium salt to the polymer composition are made to realize a lower electric resistance. However it is difficult to obtain a volume resistivity value lower than 107.8 or 107.9. The additive for increasing the ionic conductivity may stain the photosensitive member. Further in a system in which the electric resistance is dropped by the additive, the electric resistance rise much at the time of a continuous energization.
The acrylonitrile-butadiene rubber (NBR) has favorable mechanical properties such as the compression set. However, the electric resistance of the acrylonitrile-butadiene rubber is more than 109.5 [Ω·cm] and thus higher than that of the urethane. Even though an additive for increasing the ionic conductivity is used, it is impossible to make the electric resistance value of the acrylonitrile-butadiene rubber lower than 108.0 [Ω·cm].
Regarding the epichlorohydrin rubber in a system in which the photosensitive member is not stained, the electric resistance value of the—epichlorohydrin rubber is not less than 108.0 [Ω·cm] which is a little lower than that of urethane. However, in the system in which the electric resistance is dropped by the additive, the electric resistance rise much at the time of the continuous energization, as in the case of the urethane.
Regarding the electric resistance value of the epichlorohydrin rubber, it is known that there is a close correlation between the copolymerization ratio of ethylene oxide (may be hereinafter referred to as EO) and the electric resistance thereof in GECO (ethylene oxide-epichlorohydrin (may be hereinafter referred to as EP)-allyl glycidyl ether (may be hereinafter referred to as (AGE)) copolymer) and ECO (ethylene oxide-epichlorohydrin copolymer).
In the proposal disclosed in Japanese Patent Application Laid-Open No. 2000-63656, the copolymerization ratio of the EO is increased to reduce the electric resistance value. More specifically, an epichlorohydrin rubber composition is allowed to have a low electric resistance of 106.9 [Ω·cm] in a sulfur crosslinking system by the increase of the copolymerization ratio of the EO.
Representative vulcanizing systems for the epichlorohydrin rubber are a sulfur vulcanizing system, a vulcanizing system using a triazine derivative, a peroxide vulcanizing system, a vulcanizing system using a derivative of 2,3 dimethyl quinoxaline, and a vulcanizing system using thiourea.
It is conceivable to reduce the electric resistance value and the extent of stain of the photosensitive member in the sulfur vulcanizing system. However, the electric resistance will increase and the compression set will deteriorate. In the peroxide vulcanizing system, the photosensitive member is allowed to have a low degree of staining by carrying out secondary vulcanization. In this case, the electric resistance value rises. Thus, the use of the peroxide vulcanizing system is unpractical. The vulcanizing system using the derivative of 2,3 dimethyl quinoxaline has an advantage of outstandingly reducing the compression set but stains the photosensitive member much and modifies members on the periphery thereof. Thus the vulcanizing system using the derivative of the 2,3 dimethyl quinoxaline is not of practical use.
The vulcanizing system using the thiourea is capable of realizing a low electric resistance, a low compression set, and a low extent of staining of the photosensitive member, unlike the other vulcanizing systems. Therefore many vulcanizing systems use the thiourea. But in the vulcanizing system using the thiourea, lead oxide such as trilead tetraoxide is used as a acid accepting agent and an activator in a crosslinking reaction.
As the vulcanizing system using the thiourea, a semi-conductive rubber material containing the epichlorohydrin rubber and 2-mercaptoimidazoline is disclosed in Japanese Patent Application Laid-Open No. 6-242667.
In the case where a polymer composition such as rubber is used to form a charging roller, a developing roller, a toner supply roller, a transfer roller, and a transfer belt for use in a copying machine and a printer, there are proposed an organic peroxide-containing system, a thioureas-containing system, crosslinking (vulcanizing) systems such as a sulfur crosslinking (vulcanizing) system in consideration of a high productivity and various properties necessary for rollers. For example, the sulfur vulcanizing system is used because it provides a high vulcanizing speed and preferable property.
More specifically, in Japanese Patent Application Laid-Open No. 2000-212330, there is disclosed a vulcanized rubber composition containing a mixture of nitrile rubber and epichlorohydrin rubber and two or more vulcanizing agents selected from a group of a sulfur vulcanizing agent, a triazine compound, and a 2,3-dimelcaptoquinoxaline compound added thereto.
Disclosed in U.S. Pat. No. 3,121,163 is a conductive roller having a resistance-adjusting layer containing the epichlorohydrin rubber, a specific age resistor, and 2-melcaptoimidazoline (ethylene thiourea).
In order for the conductive roller to display a sufficient function, it is necessary to make a nip width large to some extent. To do so, a plasticizer such as dibutyl phthalate (DBP), dioctyl phthalate (DOP) or tricresil phosphate is added to a rubber composition for a conductive roller or a foaming agent is used together with a urea assistant foaming agent to foam the rubber composition and make its hardness very low.
However, in the epichlorohydrin rubber composition having a low electric resistance value in the sulfur vulcanizing system disclosed in Japanese Patent Application Laid-Open No. 2000-63656, merely the prescription of the general-purpose sulfur and the vulcanizing accelerator is utilized. Therefore it is difficult to use the epichlorohydrin rubber composition for a charging roller or a developing roller which contacts the photosensitive member directly. Further since the epichlorohydrin rubber composition has a high compression set, there occurs problems in the durability and precision maintenance of an image-forming process system including the roller.
Disclosed in Japanese Patent Application Laid-Open No. 6-242667 is the semi-conductive rubber material to be vulcanized by the thiourea serving as the vulcanizing agent. In the semi-conductive rubber material, the content of the EO of the epichlorohydrin rubber is as small as 41% and that of the mercaptoimidazoline is as small as 1.2 phr (0.0118 mol for 100 g of polymer). Thus the semi-conductive rubber material has a high electric resistance value and stains the photosensitive member much.
It is conceivable to use the ethylene thiourea and hydrotalcite as the vulcanizing system using the thiourea. However, the use of the ethylene thiourea and the hydrotalcite does not have a sufficient crosslinking efficiency. Thus the rubber composition stains the photosensitive member and does not have a low compression set. Therefore the rubber composition is not of practical use.
Accordingly used currently are a carbon-containing electroconductive polymer composition and urethane rubber or epichlorohydrin rubber to which electroconductivity is imparted by carbon in the range in which ionic conductivity is not deteriorated. However, in a product and among products, the system using the ionic-conductive polymer composition and the electroconductive polymer composition in combination has a larger variation in the electric resistance than the ionic-conductive polymer composition. Further the former depends on a voltage to a much higher extent than the latter. Furthermore the former has problems in preventing the photosensitive member from being stained and realizing a low compression set. The system using the ionic-conductive polymer composition and the electroconductive polymer composition in combination has not been realized in the mode in which it can be put into practical use.
The present invention has been made in view of the above-described problems. Therefore it is a first object of the present invention to provide a high-performance polymer composition having a low volume resistivity value and a low compression set and not staining a photosensitive member. It is also a first object to provide a conductive roller composed of the polymer composition and not polluting environment and having a high operability. The conductive roller includes a developing roller, a charging roller, and a transfer roller for use in a color copying machine and a color printer.
In the vulcanized rubber composition disclosed in Japanese Patent Application Laid-Open No. 2000-212330, the nitrile rubber (NBR) and the epichlorohydrin rubber not having chlorine in molecules thereof are mixed with each other at the weight ratio of 20/80-80/20. Thus the crosslinking density of the NBR that is crosslinked with only sulfur and sulfur-vulcanizing accelerator becomes low, and the photosensitive member is stained. This is because the sulfur and the sulfur-vulcanizing accelerator are also distributed to and consumed by the epichlorohydrin rubber. Blooming may occur if the amount of the sulfur and that of the sulfur-vulcanizing accelerator are increased to prevent this. If the crosslinking density increases unnecessarily to prevent the photosensitive member from being stained, the electric resistance value will become higher. Further if a can vulcanization is made when the amount of a filler is small in the vulcanizing system using only sulfur, it may be necessary to set a mixture of components in a receiving die before vulcanization is made in consideration of a heat-caused deformation thereof. Thus the manufacturing cost is high owing to the labor of setting the mixture in the receiving die and equipment expense and the productivity will be low.
In the crosslinking system not using sulfur and using only the thiourea disclosed in U.S. Pat. No. 3,121,163, a crosslinking reaction is slow. Thus the productivity is low. If the amount of the thioureas or the accelerator is increased much to enhance productivity, blooming or the stain of the photosensitive member may occur. Depending on a case, rubber may be burnt in a kneader or an extruder.
The present invention has been made in view of the above-described problems. Therefore it is a second object of the present invention to provide a polymer composition allowing a crosslinking speed to be high while preventing a photosensitive member from being stained and allowing reduction of a compression set. It is also a second object of the present invention provide a conductive roller and a conductive belt excellent in various properties and productivity.
In the case where the plasticizer is used, as described above to make the nip width large, the plasticizer moves to the surface of the roller and stains the photosensitive member. In the case where the polymer composition is foamed by using a urea assistant foaming agent, inhibition of vulcanization will occur. Consequently the vulcanizing speed will decrease, the productivity will be low, the strength of the foamed polymer composition will be low, and the photosensitive member may be stained in dependence on a case.
In the case where the foaming agent is used, vulcanization is performed mainly by sulfur. In this case, a scorch time takes long even though the urea assistant foaming agent is not used and inhibition of vulcanization occurs. In the case where can vulcanization (vulcanizer) is performed by using pressurized water vapor, a foamed tube will get out of shape, the inner and outer surfaces of the foamed tube will be rough, and the foamed tube will warp in its longitudinal direction. Thus the obtained foamed tube is not preferable or defective.
The present invention has been made in view of the above-described problems. Therefore it is a third object of the present invention to provide a polymer composition allowing a vulcanizing speed to be high while preventing generation of blooming, reduction of a compression set to be accomplished, not having the problem of inhibition of vulcanization, having a high strength and a preferable surface when the polymer composition is foamed, and being superior in productivity. It is also a third object of the present invention to provide a conductive roller composed of the polymer composition.
It is a fourth object of the present invention to provide a polymer composition not polluting environment and having a high operability and a conductive roller composed of the polymer composition.